In a typical installation utilizing ceramic filter elements of this general type, the particulate-laden gas stream is directed through one or more inertial separating devices such, for example, as cyclones, which remove the bulk of the particulate matter. The particle size distribution of solid material leaving the inertial separators is generally in the range of about 0.2 to 15 microns with the most commonly occurring size being about 4 microns. Such gas streams commonly have a temperature in excess of 1,600 degrees F., which accounts for the fact that porous ceramic material is currently the most durable filter material used for these applications.
A major problem associated with ceramic filters as well as with metal or plastic filters has been plugging of the filters. Other problems have been constructional in nature, i.e., the filter element itself must be resistant to thermal shock and must also be sufficiently strong to withstand rough handling during shipment, installation and overall filter maintenance.
As is known in the art, the denser the filter, i.e., the lower the mean pore size, the more effective is the filter in removing small size particles. It should be understood that the term "grade" of a porous ceramic element is commonly defined as the quantity of air (SCFM) that will pass through a 1 square foot surface area of the ceramic material with a thickness of one inch and a differential pressure of two inches of water. It may thus be seen that as the grade of the element decreases numerically so does the mean pore size of the element and so does the air flow through the filter element at a constant pressure differential. If, however, the grade of the ceramic filter section is increased, the effectiveness of the filter element to remove microscopic particles from the gas being filtered is decreased due to the increase in mean pore size. On the other hand as the pore size or "grade" of the porous ceramic element is decreased the resistance to gas flow through the filter element is increased. In addition, the resistance to gas flow through the filter element increases proportionately to the thickness of the filter element. It might appear that optimum filtration performance could be achieved by the use of a very thin and very tight filter element. However, such a filter element would inherently be too fragile for use in practical applications As a consequence, the filter elements of the prior art, whether designed for gas or liquid filtration, have been a design compromise between optimum pore size for efficient filtration and optimum thickness for strength and durability.